Such helically wound tubular membranes as well as a method and apparatus for producing them are known from for example GB-1 325 672. This publication describes helically winding on a winding section of a mandrel one or more porous fibrous tapes to produce a single or multi-ply tube, and continuously casting as a liquid dope a semi-permeable membrane onto the inside of the tube formed on the mandrel. The casting is done in situ in one go, that is to say when the tubular membrane is formed on the mandrel. For this the casting takes place along a casting section of the mandrel which adjoins the winding section of the mandrel. After the casting a continuous doctoring takes place in which the membrane forming material gets equally distributed with a desired layer thickness over the inner wall of the support tube. The doctoring is also done in situ in one go, that is to say when the tubular membrane is formed on the mandrel. For this the doctoring takes place along a doctoring section of the mandrel which adjoins the casting section of the mandrel.
The thus made helically wound tubular membranes have substantially smooth, round inner surfaces which act as separation layer and are suitable to be used in for example cross flow modules for filtration processes. Fluid to be filtered is fed to one of the outer ends of the tubular membranes. Permeate flows through the membrane layer, while solutes and/or particles present in the fluid are rejected and drained away as retentate. In such smooth tubular membranes a mainly laminar flow takes place alongside the inner surface of the membrane layer. This laminar flow is referred to as the boundary layer and does not get (well) mixed with the main flow of fluid through the tubular membrane.
The most significant limitations to filtration performances of such tubular membranes are concentration polarization near the membrane layer, the buildup of a cake along the membrane layer and fouling of the membrane layer. Concentration polarization is defined as accumulation of rejected solutes near the membrane layer, which results in higher solutes concentrations being present there. The concentrations of the rejected solutes near the membrane layer may increase up to 100 times. When the concentrations of the solutes get too high, a gel may even start to precipitate onto the membrane layer. This gel then aids to the forming of a solid cake.
The only way the rejected solutes can get away from the membrane layer is by back-diffusion down the concentration gradient. The rate of back-diffusion of the solutes out of the boundary layer into the main flow is governed by the diffusivity of those solutes, and by the thickness of the boundary layer. Since the diffusivity of the rejected solutes is physically determined, this parameter can not be influenced. The thickness of the boundary layer however can be manipulated by changing the cross flow velocity and/or by influencing the flow patterns inside the tubular membranes, like promoting local turbulences and secondary flows.
For this purpose it is already well known from the state of the art to place turbulence enhancers and secondary flow inducers inside tubular membranes in order to modify the flow patterns therein and in particular help to reduce the thickness of the boundary layer.
For example it has been known to place helical coil inserts inside tubular membranes. The windings of those coil inserts then come to lie against the membrane layer and as it were form ridges thereupon. Those ridges cause turbulences and secondary flows to occur in front and behind them, thus mixing the boundary layer and helping to minimize concentration polarization, buildup of cake and fouling along the membrane layer.
The use of such helical coil inserts however has the disadvantage that the coil inserts can only be drawn through tubular membranes after first having been elastically stretched to such an extent that they have become more slender than the inner diameter of the tubular membranes themselves. Subsequently they need to be carefully pulled through the tubular membranes while remaining in those stretched positions such that they cannot harm the vulnerable membrane layers. Only then can the coil inserts be released such that they can take back their original position and with this come to lie against the membrane layers of the tubular membranes. As one can imagine this is a time-consuming and difficult operation which cannot be performed in large numbers on an industrial scale, particularly for smaller diameter membrane tubes.
In the article “Hydrodynamic aspects of filtration antifouling by helically corrugated membranes” in the name of L. Broussous, P. Schmidtz, H. Boisson, E. Prouzet and A. Larbot in the Journal “Chemical Engineering Science” 55 (2000) 5049-5057, mention is being made of a ceramic tubular membrane geometry with a helical relief stamp at the membrane surface in order to maintain a high level of turbulence close to the surface during filtration. For the production of such a ceramic type tubular membrane, during a first step a macro-porous ceramic support tube needs to be extruded. The relief stamp is co-extruded with the ceramic support tube. For this a special extrusion head is needed with a rotating inner part. Subsequently the extruded corrugated ceramic support tube needs to be baked in an oven. Only after that, in a final step, the baked corrugated ceramic support tube needs to be provided with a membrane layer on its inner wall.
A disadvantage hereof is that a relative complex extrusion process is needed. Furthermore it is limited to ceramic support tubes, which are relative expensive to manufacture. Finally it appeared that only gradually sloping corrugations could be obtained as relief stamp at the membrane surface with this manufacturing method, leading to only a limited reduction of the boundary layers and thus only to a limited increase in performances.